WO2021077438A1

WO2021077438A1 - Large-scale photovoltaic direct-current series boosting grid-connected system having power balancers

Info

Publication number: WO2021077438A1
Application number: PCT/CN2019/113545
Authority: WO
Inventors: 黄欣科; 王环; 卢俊龙; 王一波; 国建鸿; 由弘扬; 张新雷
Original assignee: 中国科学院电工研究所
Priority date: 2019-10-25
Filing date: 2019-10-28
Publication date: 2021-04-29
Also published as: CN110867846A; US10998723B1; US20210151984A1; CN110867846B

Abstract

A large-scale photovoltaic direct-current series boosting grid-connected system having power balancers, and a power balancing method. The grid-connected system comprises N photovoltaic direct-current converters (103) and N-1 power balancers (102), N≥2; output ends of the photovoltaic direct-current converters are sequentially connected in series and then connected to a direct-current grid (105), and input ends of the photovoltaic direct-current converters are respectively connected to output ends of photovoltaic power generation units (101); in the photovoltaic direct-current converters which are sequentially provided in series, a power balancer is provided between input ends of two adjacent photovoltaic direct-current converters; and the N-1 power balancers are respectively provided corresponding to the provided N-1 photovoltaic direct-current converters, and are used for performing power equalization on inputs of corresponding photovoltaic direct-current converters, so as to eliminate differences in output voltages of the photovoltaic direct-current converters. The grid-connected system solves the problems of light abandonment and overvoltage output of the photovoltaic direct-current converters caused by mismatching of input powers of the photovoltaic direct-current converters in said system, improving the generating capacity of said system.

Description

Large-scale photovoltaic DC series boost grid-connected system with power balancer

Technical field

The invention belongs to the technical field of photovoltaic power generation, and in particular relates to a large-scale photovoltaic DC series boost grid-connected system with a power balancer.

Background technique

The development and utilization of new energy sources such as solar photovoltaic and wind power are important ways to solve the energy crisis and environmental problems. At present, there are many million-kilowatt photovoltaic bases. Traditional large-scale photovoltaic power plants usually use centralized AC to converge, boost, and merge into the AC transmission grid through medium and high voltage AC. With the large-scale and large-scale centralized grid connection of photovoltaic power stations, traditional AC grid-connected methods have problems such as harmonic resonance, inability to absorb and send out, and need large-capacity reactive power compensation, and the power conversion equipment and power required for AC grid-connected Problems such as multiple conversion links and low efficiency.

With the rapid development of medium and high-voltage flexible DC transmission, the current PV DC boost convergent access scheme is proposed, which can effectively avoid the problem of AC grid-connected transmission of traditional photovoltaic power stations, and photovoltaic DC grid-connected can reduce power conversion links, save power conversion equipment, It has obvious economic and technical advantages to reduce system cost and improve the overall efficiency of the system. Especially the photovoltaic DC series boost grid-connected system has become a research hotspot. The technical difficulty of the photovoltaic DC series boost grid-connected system is that when the input power of each photovoltaic DC converter in the series system does not match, it will cause the photovoltaic DC converter output over-voltage problem, and at the same time will cause the photovoltaic DC converter to abandon the problem.

Summary of the invention

In order to solve the above-mentioned problems in the prior art, that is, to solve the photovoltaic DC converter series boost grid-connected system in the large-scale photovoltaic power station due to the mismatch of the input power of the photovoltaic DC converters in the system, the abandonment and the output of the photovoltaic DC converter Voltage issues, one aspect of the present invention proposes a large-scale photovoltaic DC series boost grid-connected system with a power balancer. The system includes N photovoltaic DC converters, N-1 power balancers, and N≥2.

The output terminals of each photovoltaic DC converter are connected in series and then connected to the DC grid, and the input terminals of each photovoltaic DC converter are respectively connected with the output terminals of the photovoltaic power generation unit;

Among the photovoltaic DC converters arranged in series, a power balancer is arranged between the input terminals of two adjacent photovoltaic DC converters;

N-1 power balancers are respectively set corresponding to the set N-1 photovoltaic DC converters, which are used to balance the input power of the corresponding photovoltaic DC converters to eliminate the difference in output voltage of each photovoltaic DC converter .

In some preferred embodiments, the power balancer includes a control signal input terminal, a main power circuit, and a current path connection terminal;

Output voltage of the control signal input for acquiring a photovoltaic DC converter corresponding value V _i, N photovoltaic DC converter stage output voltage value of the average value V _a;

The main power circuit based on the difference between V _i and V _a control current between the two photovoltaic DC converter input terminal;

The two ends of the current path connecting ends are respectively connected to the input ends of the two corresponding photovoltaic DC converters for establishing current paths.

In some preferred embodiments, the "V _a and V _i based on a difference between the control current input terminal of two photovoltaic DC converter", which method:

When V _i> V _a, corresponding to the i-th PV DC converter power balancers generate negative current based on the difference V _a and V _i, the output current of the DC converter corresponding to the i-th PV photovoltaic power generation unit Perform shunting to reduce the input current of the i-th photovoltaic DC converter;

When V _i <V _a, corresponding to the i-th PV DC power converter forward current balancer based on a difference V _a and V _i, the output current of the i-th PV DC converter corresponding to the photovoltaic power generation unit Confluence to increase the input current of the i-th photovoltaic DC converter.

In some preferred embodiments, the power balancer is provided with a bypass circuit, and the output terminal of the photovoltaic DC converter is provided with a short circuit; when any photovoltaic DC converter fails, the bypass circuit can be used Connect its corresponding photovoltaic power generation unit to the adjacent photovoltaic DC converter connected to its corresponding power balancer, and at the same time short-circuit the output terminal of the failed photovoltaic DC converter through the short circuit to maintain each photovoltaic DC The output terminals of the converter are connected in series to the path of the DC power grid.

In some preferred embodiments, the photovoltaic DC converter adopts a dual closed-loop control strategy of input voltage and input current for maximum power tracking control.

In some preferred embodiments, the power balancer adopts closed-loop control of the output voltage of the photovoltaic DC converter.

In the second aspect of the present invention, a power balance method for a large-scale photovoltaic DC series boost grid-connected system is proposed. Based on the above-mentioned large-scale photovoltaic DC series boost grid-connected system with a power balancer, each power balancer separately The input power of the corresponding photovoltaic DC converter is balanced, including:

After the large-scale photovoltaic DC series boost grid-connected system starts normally, the output voltage value V _{i of the i-th} _{photovoltaic DC converter and the average value V a of the} output voltage values of the N photovoltaic DC converters are obtained, and the following equalization control is performed:

When V _i =V _a , the power balancer corresponding to the i-th photovoltaic DC converter does not work;

In some preferred embodiments, the photovoltaic DC converter without a power balancer always adopts the maximum power tracking closed-loop control to make it run in the maximum power tracking mode, and performs output voltage limiting control on the photovoltaic DC converter.

In the third aspect of the present invention, a photovoltaic DC power generation system is proposed, which includes N groups of photovoltaic power generation units, and also includes the aforementioned large-scale photovoltaic DC series boost grid-connected system with a power balancer.

The beneficial effects of the present invention:

The invention can solve the problem of light abandonment and overvoltage caused by the mismatch of the input power of each photovoltaic DC converter in the photovoltaic DC series boost grid-connected system; solves the problem of the mismatch of the input power of each photovoltaic DC converter The problem of reduced system power generation caused by increased system power generation.

Description of the drawings

By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, purposes, and advantages of the present application will become more apparent:

Figure 1 is a schematic diagram of a large-scale photovoltaic DC series boost grid-connected system with a power balancer according to an embodiment of the present invention;

Figure 2 is a control block diagram of a photovoltaic DC converter in an embodiment of the present invention;

Fig. 3 is a control block diagram of a power balancer in an embodiment of the present invention;

Fig. 4 is a schematic diagram of a large-scale photovoltaic DC series boost grid-connected system with a power balancer when N=3 in an embodiment of the present invention.

Detailed ways

In order to make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not All examples. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention.

The application will be further described in detail below with reference to the drawings and embodiments. It can be understood that the specific embodiments described here are only used to explain the related invention, but not to limit the invention. In addition, it should be noted that, for ease of description, only the parts related to the relevant invention are shown in the drawings.

It should be noted that the embodiments in this application and the features in the embodiments can be combined with each other if there is no conflict.

A large-scale photovoltaic DC series boost grid-connected system with power balancer of the present invention includes N photovoltaic DC converters, N-1 power balancers, N≥2; the output terminals of each photovoltaic DC converter are connected in series in sequence After connecting to the DC grid, the input terminals of each photovoltaic DC converter are independent of each other, and are respectively connected to the output terminal of the corresponding photovoltaic power generation unit; among the photovoltaic DC converters arranged in series, the two adjacent photovoltaic DC converters are connected in series. A power balancer is set between the input terminals; N-1 power balancers are respectively set corresponding to the set N-1 photovoltaic DC converters, and are used to balance the input power of the corresponding photovoltaic DC converters to eliminate The difference of the output voltage of each photovoltaic DC converter.

The photovoltaic power generation unit in the present invention can be a photovoltaic module, or a photovoltaic string or a photovoltaic array. In this embodiment, the photovoltaic power generation unit is a photovoltaic array. As shown in FIG. 1, N sets of photovoltaic arrays 101 are respectively arranged in a one-to-one correspondence with N sets of photovoltaic DC converters 103, and the N sets of photovoltaic DC converters 103 are connected in series after passing through high voltage. The DC line 104 is connected to the DC power grid 105, a power balancer 102 is arranged between two adjacent photovoltaic DC converters 103, and the power balancer 102 is respectively connected to the input ends of the two adjacent photovoltaic DC converters 103.

As shown in Figure 2, in a large-scale photovoltaic DC series boost grid-connected system with a power balancer, each photovoltaic DC converter detects the output voltage V _PV and output current I _{PV of} its corresponding photovoltaic array in real time, and then passes the maximum power The tracking controller (MPPT) implements the maximum power tracking algorithm, given the MPPT reference voltage V _PV-ref , and then compares the actual output voltage of the photovoltaic array V _PV with the given MPPT reference voltage V _PV-ref to obtain the voltage difference V _{PV- e} . After passing the voltage controller, the PV DC converter input current reference value I _in ^{* is given} , and then the actual input current I _{in of the} PV DC converter is compared with the input current reference value I _in ^* to obtain the current difference I _{in_e} , The control duty ratio D of the photovoltaic DC converter is given through the current controller, and finally the maximum power tracking double closed-loop control of the photovoltaic DC converter is realized. FIG. 2 shows the photovoltaic DC converter MPPT voltage control loop 201, the photovoltaic DC converter input current control loop 202, the MPPT control algorithm module 203, the MPPT voltage controller 204, and the photovoltaic DC converter input current controller 205.

Power balancer includes a control signal input terminal, a main power circuit, a current path connecting terminal; controlling an output voltage value of the input signal V _i for acquiring the corresponding photovoltaic DC converter, the mean value of the inverter output voltage DC photovoltaic units N V _a ; The main power circuit is used to _{control the current between the input ends of the two photovoltaic DC converters based on the difference between Vi} and V _a; the two ends of the current path connection ends are respectively corresponding to the input ends of the two photovoltaic DC converters Connection, used to establish a current path. As shown in Figure 3, in the i-th power balancer, the corresponding i-th photovoltaic DC converter detects its output voltage V _{i in} real time, and the average value of the output voltage of all photovoltaic DC converters V _a (V _a = V _s /N, where V _s is the sum of the output voltage values of all photovoltaic DC converters) to obtain the voltage difference V _{i_e} , and then give the control duty cycle d of the power balancer through the voltage controller, and then control the The conduction direction and current magnitude of the current in the power balancer. Fig. 3 shows the output voltage control loop 301 of the photovoltaic DC converter and the output voltage controller 302 of the photovoltaic DC converter.

When the input power of each photovoltaic DC converter in the series system is equalized, each photovoltaic DC converter can normally achieve maximum power tracking, running in the maximum power tracking mode, and almost no current flows in the power balancer; when in the series system When the input power of each photovoltaic DC converter is unbalanced, each photovoltaic DC converter can still achieve maximum power tracking normally and run in maximum power tracking mode. At this time, by detecting the actual output voltage and average output of each photovoltaic DC converter The relationship between voltage, control the work of the power balancer, there will be current flowing in the power balancer, the direction of the current is determined by the relationship between the actual output voltage of the photovoltaic DC converter and the average output voltage, so that each photovoltaic in the series system can be realized. The input power of the DC converter is balanced, and the output voltage of the photovoltaic DC converter is maintained at the average output voltage.

DC current between the two photovoltaic converter input control based on a difference V _i and V _a power equalizer in which method:

In this embodiment, a bypass circuit is provided in the power balancer, and a short circuit is provided at the output of the photovoltaic DC converter; when any photovoltaic DC converter fails, the corresponding photovoltaic DC converter can be The power generation unit is connected to the adjacent photovoltaic DC converter connected to its corresponding power balancer, and at the same time, the output terminal of the failed photovoltaic DC converter is short-circuited through the short circuit to maintain the output terminal of each photovoltaic DC converter Connect to the DC grid in series. In this way, the photovoltaic array can continue to output power.

In this embodiment, the photovoltaic DC converter adopts a dual closed-loop control strategy of input voltage and input current for maximum power tracking control. The power balancer adopts closed-loop control of the output voltage of the photovoltaic DC converter.

In this embodiment, all photovoltaic DC converters must limit the output voltage, and all power balancers must limit the current to ensure the equipment safety of the photovoltaic DC converters and power balancers.

The control of the large-scale photovoltaic DC series boost grid-connected system with power balancer of the present invention can be divided into two parts. One part is the control of each photovoltaic DC converter in the series system. The controllers of all photovoltaic DC converters in the series system are It can be regarded as independent of each other, mainly realizing the maximum power tracking control of the corresponding photovoltaic array, adopting the dual closed-loop control strategy of input voltage and input current; the other part is the control of N-1 power balancers in the series system, and the power of each unit is balanced. The controller is mainly controlled by the relationship between the real-time output voltage of the corresponding photovoltaic DC converter and its average output voltage, and the closed-loop control of the output voltage of the photovoltaic DC converter is adopted. Thus, the adaptive coordinated control of N photovoltaic DC converters and N-1 power balancers in the series system can be realized.

Figure 4 shows an example of a large-scale photovoltaic DC series boost grid-connected system with a power balancer when N=3. The system includes 3 photovoltaic arrays, 3 photovoltaic DC converters, and 2 power Balancer; a power balancer 1 is provided between the input terminals of the photovoltaic direct current converter 1 and the photovoltaic direct current converter 2, and a power balancer 2 is provided between the photovoltaic direct current converter 2 and the input end of the photovoltaic direct current converter 3; 3 photovoltaics The output terminals of the DC converters are connected in series after being connected to the DC grid. In the system shown in the figure, the output voltage of the photovoltaic DC converter 3 is lower than the average value of the output voltages of all photovoltaic DC converters, and the power balancer 1 diverts the current I ₁₁ from the input end of the photovoltaic DC converter 1 and divides the first The output current I _{PV1 of} a photovoltaic array is changed to I _in1 as the input of the photovoltaic DC converter 1, and the current I _{12 is} _{merged into the output current I PV2} of the photovoltaic array corresponding to the photovoltaic DC converter 2 after the power balancer 1; _{The power balancer 2 divides the current I 21} from the input terminal of the photovoltaic DC converter 2 and changes the combined current value of the output current I _PV1 and I _{12 of the} _{first photovoltaic array to I in2} as the input of the photovoltaic DC converter 2 _{, And merge the current I 22} _{into the output current I PV3} of the photovoltaic array corresponding to the photovoltaic DC converter 3 after the power balancer 2 to obtain the input current I _in3 of the photovoltaic DC converter 3; the output voltages of the three photovoltaic DC converters They are V ₁ , V ₂ , V ₃ , and the combined voltage is V _S.

The power balance method of a large-scale photovoltaic DC series boost grid-connected system according to an embodiment of the present invention is based on the above-mentioned large-scale photovoltaic DC series boost grid-connected system with a power balancer. After the system meets the starting conditions and starts, each photovoltaic The DC converter gradually enters the maximum power tracking mode, and each power balancer balances the input power of its corresponding photovoltaic DC converter;

Take the i-th photovoltaic DC converter and its corresponding i-th power balancer as an example to obtain the output voltage value V _{i of the i-th} _{photovoltaic DC converter and the average value V a of the} output voltage values of the N photovoltaic DC converters, And perform the following balance control:

(1) When V _i =V _a , the photovoltaic DC converter works in the maximum power tracking mode, and the power balancer corresponding to the i-th photovoltaic DC converter does not work, that is, the i-th power No current flows in the balancer;

(2) When V _i >V _a , the photovoltaic DC converter is working in the maximum power tracking mode, and there is at least one photovoltaic DC converter in the series system whose output voltage is less than the average output voltage of the converter, that is, in the series system each stage photovoltaic inverter input DC power is not balanced, this time, the i-th power balancer work, have a negative current based on the difference of V _a and V _i, for the i-th PV DC converter corresponding photovoltaic cell The output current is shunted to reduce the input current of the i-th photovoltaic DC converter, thereby reducing the input power and output voltage of the photovoltaic DC converter, and the compensation input power and output voltage are lower than the average input power and average output of the converter Voltage photovoltaic DC converters, finally realize the balance of input power and output voltage of all photovoltaic DC converters in the series system.

(3) When V _i <V _a , the photovoltaic DC converter works in the maximum power tracking mode, and there is at least one photovoltaic DC converter in the series system whose output voltage is greater than the average output voltage of the converter, that is, in the series system each stage photovoltaic inverter input DC power is not balanced, this time, the i-th power balancer to work, based on the difference between the forward current and V _i V _a, and the i-th PV DC converter corresponding photovoltaic cell The output currents are combined to increase the input current of the i-th photovoltaic DC converter, thereby increasing the input power and output voltage of the photovoltaic DC converter, and eliminating the input power and output voltage is greater than the average input power and average output voltage of the converter The photovoltaic DC converters finally realize the equalization of the input power and output voltage of all photovoltaic DC converters in the series system.

The photovoltaic DC converter without a power balancer always adopts the maximum power tracking closed-loop control to make it run in the maximum power tracking mode, and performs output voltage limiting control on the photovoltaic DC converter to ensure the safety of the photovoltaic DC converter.

A photovoltaic DC power generation system according to an embodiment of the present invention includes N groups of photovoltaic power generation units, and also includes the above-mentioned large-scale photovoltaic DC series boost grid-connected system with a power balancer.

Those skilled in the art can clearly understand that for the convenience and brevity of the description, the specific working process and related instructions of the photovoltaic DC power generation system described above can refer to the aforementioned large-scale photovoltaic DC series boost and grid connection with power balancer The corresponding process in the system embodiment will not be repeated here.

The term "including" or any other similar term is intended to cover non-exclusive inclusion, so that a process, method, article or device/device including a series of elements includes not only those elements, but also other elements not explicitly listed, or It also includes the inherent elements of these processes, methods, articles, or equipment/devices.

So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

A large-scale photovoltaic DC series boost grid-connected system with a power balancer, characterized in that the system includes N photovoltaic DC converters, N-1 power balancers, and N≥2;

The output terminals of each photovoltaic DC converter are connected in series and then connected to the DC grid, and the input terminals of each photovoltaic DC converter are respectively connected with the output terminals of the photovoltaic power generation unit;

Among the photovoltaic DC converters arranged in series, a power balancer is arranged between the input terminals of two adjacent photovoltaic DC converters;

N-1 power balancers are respectively set corresponding to the set N-1 photovoltaic DC converters, which are used to balance the input power of the corresponding photovoltaic DC converters to eliminate the difference in output voltage of each photovoltaic DC converter .
The large-scale photovoltaic DC series boost grid-connected system with a power balancer according to claim 1, wherein the power balancer comprises a control signal input terminal, a main power circuit, and a current path connection terminal;

Output voltage of the control signal input for acquiring a photovoltaic DC converter corresponding value V i, N photovoltaic DC converter stage output voltage value of the average value V a;

The main power circuit based on the difference between V i and V a control current between the two photovoltaic DC converter input terminal;

The two ends of the current path connecting ends are respectively connected to the input ends of the two corresponding photovoltaic DC converters for establishing current paths.
The large-scale photovoltaic power having a DC balancer according to claim 2 in series boosted grid system, characterized in that the "V a and V i based on a difference between the control current input terminal of two photovoltaic DC converter" , The method is:

When V i> V a, corresponding to the i-th PV DC converter power balancers generate negative current based on the difference V a and V i, the output current of the DC converter corresponding to the i-th PV photovoltaic power generation unit Perform shunting to reduce the input current of the i-th photovoltaic DC converter;

When V i <V a, corresponding to the i-th PV DC power converter forward current balancer based on a difference V a and V i, the output current of the i-th PV DC converter corresponding to the photovoltaic power generation unit Confluence to increase the input current of the i-th photovoltaic DC converter.
The large-scale photovoltaic DC series boost grid-connected system with a power balancer according to claim 1, wherein the power balancer is provided with a bypass circuit, and the output terminal of the photovoltaic DC converter is provided with a short circuit. Circuit; when any photovoltaic DC converter fails, its corresponding photovoltaic power generation unit can be connected to the adjacent photovoltaic DC converter connected to its corresponding power balancer through the bypass circuit, and at the same time through the short circuit The circuit short-circuits the output terminal of the faulty photovoltaic DC converter to maintain the path of connecting the output terminals of the photovoltaic DC converters in series to the DC power grid.
The large-scale photovoltaic DC series boost grid-connected system with a power balancer according to any one of claims 1 to 4, wherein the photovoltaic DC converter adopts a dual closed-loop control strategy of input voltage and input current for maximum power Tracking control.
The large-scale photovoltaic DC series boost grid-connected system with a power balancer according to any one of claims 1 to 4, wherein the power balancer adopts closed-loop control of the output voltage of the photovoltaic DC converter.
A power balance method for a large-scale photovoltaic DC series boost grid-connected system, which is characterized in that, based on the large-scale photovoltaic DC series boost grid-connected system with a power balancer according to any one of claims 1-6, each power is balanced The inverters respectively balance the input power of their corresponding photovoltaic DC converters, including:

After the large-scale photovoltaic DC series boost grid-connected system starts normally, the output voltage value V i of the i-th photovoltaic DC converter and the average value V a of the output voltage values of the N photovoltaic DC converters are obtained, and the following equalization control is performed:

When V i =V a , the power balancer corresponding to the i-th photovoltaic DC converter does not work;

When V i> V a, corresponding to the i-th PV DC converter power balancers generate negative current based on the difference V a and V i, the output current of the DC converter corresponding to the i-th PV photovoltaic power generation unit Perform shunting to reduce the input current of the i-th photovoltaic DC converter;

When V i <V a, corresponding to the i-th PV DC power converter forward current balancer based on a difference V a and V i, the output current of the i-th PV DC converter corresponding to the photovoltaic power generation unit Confluence to increase the input current of the i-th photovoltaic DC converter.
The power balance method of a large-scale photovoltaic DC series boost grid-connected system according to claim 7, characterized in that the photovoltaic DC converter without a power balancer always adopts maximum power tracking closed-loop control to make it run at maximum power tracking Mode, and perform output voltage limiting control on the photovoltaic DC converter.
A photovoltaic DC power generation system, comprising N groups of photovoltaic power generation units, characterized in that it also includes the large-scale photovoltaic DC series boost grid-connected system with a power balancer according to any one of claims 1-6.